This invention relates to providing a protective overcoat on a photographic element by using a fuser belt.
Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver.
In color photographic elements a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye forming compound called a coupler. The silver and unreacted silver halide, are then removed from the photographic element, leaving a dye image.
In either case, formation of the image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin and similar natural or synthetic hydrophilic polymers have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, the imaged element can be easily marked by fingerprints, scratched or torn, and it can swell or otherwise deform when it is contacted with liquids.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A series of patents describes methods of solvent coating a protective layer on the image after photographic processing is completed and are described in U.S. Pat. Nos. 2,259,009; 2,331,746; 2,798,004; 3,113,867; 3,190,197; 3,415,670; and 3,733,293. The application of UV-polymerizable monomers and oligomers on processed image followed by radiation exposure to form cross-linked protective layer is described U.S. Pat. Nos. 4,092,173; 4,171,979; 4,333,998; and 4,426,431. One drawback for the solvent coating method and the radiation cure method is the health and environmental concern of those chemicals to the coating operator. U.S. Pat. Nos. 3,397,980; 3,697,277; and 4,999,266 describe methods of laminating polymeric sheet film on the processed image as the protective layer. U.S. Pat. No. 5,447,832 describes the use of a protective layer containing mixture of high and low Tg latices as the water-resistance layer to preserve the antistat property of the V2O5 layer through photographic processing. This protective layer is not applicable to the image formation layers since it will detrimentally inhibit the photographic processing. U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the emulsion, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. U.S. Pat. No. 3,443,946 provides a roughened (matte) scratch-protective layer, but not a water-impermeable one. U.S. Pat. No. 3,502,501 provides protection against mechanical damage only; the layer in question contains a majority of hydrophilic polymeric materials, and must be permeable to water in order to maintain processability. U.S. Pat. No. 5,179,147 likewise provides a layer that is not water-protective.
U.S. Pat. No. 5,856,051 describes an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require coating operation after exposure and processing. This was accomplished by applying a coating comprising hydrophobic polymer particles having an average size of 0.01 to 1 microns to the silver halide light-sensitive emulsion layer. The silver halide light-sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat. This patent did not however describe the composition of any suitable materials for fusing the hydrophobic polymer particles to form the protective layer.
One key requirement of the method for fusing the particles comprising the protective overcoat is that the desired gloss level of the original unprotected photographic element be maintained. In the field of electrophotography, belt fusers have been shown to yield images with gloss values comparable to photographic elements. The belt in the belt fusing system can be made of stainless steel or polyester and the outer surface of the fuser member can be aluminum, steel, various alloys, or polymeric materials, such as, thermoset resins and fluoroelastomers.
The background art of electrophotography discloses several broad classes of materials useful for fuser belts. For example, U.S. Pat. Nos. 5,089,363; 5,465,146; 5,386,281; 5,362,833; 5,529,847; 5,330,840; 5,233,008; 5,200,284; and 5,124,755 disclose fuser belt systems consisting of belts coated with silicone polymers. U.S. Pat. No. 5,089,363 discloses that metal belts coated with highly cross-linked polysiloxanes provide fused toner images having high gloss. U.S. patent application Ser. No. 09/299,291 discloses a fusing belt can be prepared by a highly cross-linked silicone resin, but it has been found that the highly cross-linked silicone resin is brittle and may crack when the fusing belt is repeatedly flexed. Therefore, there is still need for an improved belt coating formulation for forming a protective overcoat on a photographic element.
Commonly-assigned U.S. Pat. No. 5,804,341 describes an electrostatically bound water-resistant protective overcoat that can be attached into the finished photographic product. This was accomplished by electrostatically binding a coating comprising hydrophobic polymer particles having an average size of 3 to 10 microns on to the silver halide light-sensitive emulsion layer after silver halide light sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat.
Through the recent advances in the development of protective overcoats for photographic elements, further materials are required to fuse the particulate polymers composing the protective overcoats described in U.S. Pat. Nos. 5,856,051 and 5,804,341. The prior art does not however describe the composition of any suitable materials for fusing the hydrophobic polymer particles to form the protective layer.
The present invention provides a fuser belt comprising a substrate and a coating on the substrate, the coating comprising a resin made by curing a composition comprising siloxanes having a ratio of difunctional to trifunctional units of 1:1 to 1:2.7 and at least 90% of total number of functional units of the siloxanes are difunctional and trifunctional units, a weight average molecular weight of 5,000 to 50,000, and an alkyl to aryl ratio of 1:0.1 to 1:1.2; and, coated on the intermediate layer, a surface layer that comprises a silsesquioxane polymer.
In an alternative embodiment, Although the described cross-linked silicone resin has excellent properties as an adhesive layer between the polyimide substrate and the silsesquioxane surface layer of the belt, the present invention discloses an primer adhesion promoter to avoid the inherent brittle properties of the highly cross-linked silicone resin. For example, epoxy primer can be applied to the substrate, prior to the application of the release coating. Examples of commercially available primers are W-66, epoxy primer from Emerson and Cuming Co.
In certain embodiments, the silsesquioxane layer can be directly applied to the substrate. It provides some degree of adhesion desired to the substrate layer without the cost of applying intermediate layer.
This fuser belt provides high gloss, long life, and good release of the fuser for heat-fixing a heat-softenable polymer, which is a protective overcoat for a photographic element. The protective overcoat is formed by the steps of providing a photographic element having at least one silver halide light-sensitive emulsion layer; applying a coating comprising hydrophobic polymer particles having an average size of 0.01 to 1 microns, over the at least one silver halide light-sensitive emulsion layer. The silver halide light-sensitive emulsion layer is developed to provide an imaged photographic element. The hydrophobic polymer particles are then fused to form a protective overcoat. In an alternate method, hydrophobic polymer particles having an average size of 3 to 10 microns are electrostatically bound to the outer emulsion layer.